1. Field of the Invention
The present invention relates generally to the construction and use of vascular catheters and, more particularly, to catheters which may be used both as a guidewire for positioning larger catheters and as an ultrasonic imaging catheter.
Arteriosclerosis, also known as atherosclerosis, is a common human ailment arising from the deposition of fatty-like substances, referred to as atheroma or plaque, on the walls of blood vessels. Such deposits occur both in peripheral blood vessels that feed the limbs of the body and coronary blood vessel which feed the heart. When deposits accumulate in localized regions of a blood vessel, blood flow is restricted and the person's health is at serious risk.
Numerous approaches for reducing and removing such vascular deposits have been proposed, including balloon angioplasty, where a balloon-tipped catheter is used to dilatate a region of atheroma; atherectomy, where a blade or other cutting element is used to sever and remove the atheroma; and laser angioplasty, where laser energy is used to ablate at least a portion of the atheroma. In addition to such therapeutic approaches, a variety of techniques for transluminal imaging of atheroma and other diseased regions of a blood vessel has been proposed, including endoscopic imaging techniques and ultrasonic imaging techniques. Common to all such techniques is the use of an intravascular catheter which is positioned at a desired location within the blood vessel to be treated or imaged.
Two alternative approaches may generally be employed to achieve such positioning. In the first approach, the vascular catheter is provided with a "fixed guidewire" secured to its distal end. The fixed guidewire is typically a coiled spring or other elongate resilient member having a preformed, curved tip. The catheter can then be guided through branches within the vascular network by rotating the entire catheter, causing the tip of the guidewire to enter a desired branch as the catheter is moved forward. In the second technique, an entirely separate "movable guidewire" is employed. The movable guidewire is itself a coiled spring or other resilient elongate member and will generally include a curved tip similar to that provided on the fixed guidewires described above. The vascular catheter being positioned includes a guidewire lumen which generally extends down the center of the entire length of the catheter and is sized to receive the movable guidewire. The movable guidewire is first positioned within the vascular system so that its distal end extends beyond the region of interest, and the intravascular catheter is then inserted over the movable guidewire using the guidewire lumen. Such procedures using movable guidewires are commonly referred to as "over-the-wire" insertional techniques.
Recently, ultrasonic imaging catheters have been developed for use in conjunction with various interventional therapies, including angioplasty, atherectomy, laser ablation, and the like. By imaging a diseased region prior to therapy, the treatment can be more precisely directed to both enhance effectiveness and reduce deleterious side effects. Such ultrasonic imaging catheters have generally been introduced prior to therapy, typically using a movable guidewire as just described or a fixed-tip guidewire which is secured to the forward end of the imaging catheter. After imaging has been completed, it has generally been necessary to remove the imaging catheter prior to therapy, although in some cases it has been proposed to provide a particular interventional capability on the imaging catheter itself.
The need to remove the imaging catheter before employing conventional interventional catheters is time consuming and increases the risk that the blood vessel wall will be damaged or that emboli will be accidentally dislodged. Moreover, once removed, the imaging catheter is no longer available for imaging during the therapeutic procedure unless another time-consuming catheter-exchange procedure is employed.
For these reasons, it would be desirable to provide ultrasonic imaging catheters which may also serve as a guidewire for the introduction of larger interventional catheters. Preferably, such imaging guidewire catheters should employ a rotating ultrasonic imaging system where an ultrasonic transducer, a reflective surface, or both, are rotated in order to sweep a continuous ultrasonic signal about the interior of the blood vessel wall.
2. Description of the Background Art
PCT application WO 89/07419 discloses a miniature ultrasonic imaging probe where an ultrasonic transducer is attached to the distal end of a coaxial cable within a holder. The transducer and an acoustic reflector are rigidly maintained within the holder. The ultrasonic transducer cannot be moved within the probe, and it would be necessary to rotate the entire transducer in order to image an annular section of a blood vessel. Such rotation would be deleterious to the blood vessel wall. Probes of the type described in WO 89/07419 are employed inside a protective sheath that remains stationary and covers the entire probe during imaging procedures where the probe is rotated. U.S. Pat. No. 4,794,931, describes an ultrasonic imaging catheter where a rotating transducer, or a rotating mirror in combination with a fixed transducer, is mounted within a housing at the distal end of the catheter. Guidewire catheters are described in various patents, including U.S. Pat. Nos. 4,747,406; 4,724,846; 4,682,607; and 3,416,531. Copending application Ser. No. 07/500,818, the disclosure of which is incorporated herein by reference, describes drive cables suitable for use with catheters having rotating distal work elements, where the flexibility of the cable is increased near its distal end.